Influenza remains a major health concern and an important disease of humans and animals. Influenza virus infection causes widespread morbidity and mortality worldwide, for example, in young children, the elderly, and the chronically ill. An estimated 5-15% of the global population is infected by influenza annually, causing severe illness in 3-5 million people and 250,000-500,000 deaths worldwide. In the United States, about 36,000 people die each year from influenza infection. Moreover, there have been about three influenza pandemics in each century for the last 300 years. The 1918 Spanish flu pandemic caused by Influenza A virus subtype H1N1 resulted in about 40 million deaths in a single year. The 1957 Asian flu caused by Influenza A virus subtype H2N2 and the 1968 Hong Kong flu caused by Influenza A virus subtype H3N2 also resulted in about 2 million and about 1 million deaths, respectively. The 2009 H1N1 influenza outbreak (the swine flu) is the most recent pandemic, causing substantial economic burdens globally (e.g., hospitalization, loss of productivity, disruption of travel). As of February 2010, more than 213 countries and overseas territories or communities have reported laboratory confirmed cases of pandemic H1N1 influenza, including at least 16,455 deaths worldwide. In addition, highly lethal and virulent influenza viral strains, such as the Influenza A virus subtype H5N1 (also known as avian influenza or bird flu), pose a serious threat of a potential pandemic in humans in the event that the viral mutation and reassortment lead to a strain that can be efficiently transmitted between humans.
Influenza viruses are segmented negative-strand ribonucleic acid (RNA) viruses. It consists of an internal core of segmented RNA associated with nucleoprotein, surrounded by a viral envelope with a lipid bilayer structure and external glycoproteins. The segmented nature of the viral genome allows the genetic reassortment (exchange of genome segments) to take place during mixed infection of a cell with different viral strains. In contrast to other infections, influenza undergoes continuous antigenic change and reassortment with animal reservoir strains. The inner layer of the viral envelope is composed predominantly of matrix proteins and the outer layer mostly of host-derived lipid material. The surface glycoproteins neuraminidase (NA) and hemagglutinin (HA) appear as spikes at the surface of the viral particles. These surface proteins, particularly the HA protein, determine the antigenic specificity of the influenza virus.
Influenza viruses are divided into three types, type A, B and C, based upon differences in internal antigenic proteins. The Influenza A virus may be further classified into various subtypes according to the different HA and NA viral proteins displayed on the surface of the virus. Each subtype of virus can mutate into a variety of strains with differing pathogenic profiles. Currently, there are 16 known HA antigen subtypes (H1 to H16) and 9 known NA antigen subtypes (N1 to N9). Influenza A viruses can infect humans, birds, pigs, horses, and other animals. A subset of Influenza A virus subtypes, including but not limited to, H1N1, H2N2, H3N2, H5N1, H7N7, H1N2, H9N2, H7N2, H7N3, and H10N7 subtypes, have been confirmed to infect humans. All combinations of the 16 HA and 9 NA subtypes have been identified in avian species. In addition, Influenza B virus and Influenza C virus can also infect humans.
Due to viral recombination, prior immunity to one strain does not necessarily confer protection to the next. Upon infection, a new virus replicates unchecked, while the host mounts a highly inflammatory primary immune response. An influenza infection produces an acute set of symptoms including headache, cough, sore throat, rhinitis, fever and general malaise. In severe cases or situations involving pre-existing pulmonary or cardiovascular disease, hospitalization is required. Pneumonia due to direct viral infection or due to secondary bacterial or viral invasion is the most frequent complication.
The outcome of influenza infection is dependent on both the virus and the host. The genetic makeup of the HA and NA genes confers virulence. For example, introduction of HA and NA genes from pandemic H5N1 strains to a relatively mild virus transforms the virus into a highly virulent strain in mice. During replication, Influenza virus utilizes host protein production machinery and as a result, causes death of the infected cell (cytopathology). Such respiratory epithelial cell destruction produces an array of alarm signals initiating an inflammatory reaction (cytokine cascade) that promotes the recruitment of inflammatory cells (e.g., neutrophils and CD4+/CD8+ T cells) to the delicate surface of the lung, leading to consolidation of air spaces and a decline in arterial oxygen saturation. In eliminating the virus, the host response causes further respiratory cell death, and the responding inflammatory cells (e.g., T cells) produce an additional battery of inflammatory mediators (e.g., TNFα and IFNγ), which in excess lead to a cytokine storm, causing capillary leak and resulting in pulmonary edema and leukocyte transudation into the airspaces, thereby initiating the acute respiratory distress syndrome (ARDS). More chronic symptoms of disease, such as cachexia, fever and appetite suppression, are directly linked to the concentration of systemic mediators/cytokines that accumulate. Therefore, the whole cascade is initiated by virus-induced cytopathology, but mortality is ultimately determined by the magnitude of the inflammation that results from the immune response. Finally, both the viral-induced cytopathology and the host inflammatory response can predispose the infected subject to secondary bacterial infection, further increasing morbidity and mortality. There is evidence that a reduction of inflammation can in some circumstances have a beneficial effect, but if the immune suppression is too great then viral induced cytopathology overwhelms the host.
Flu vaccination has been a somewhat effective measure to limit influenza morbidity. However, a vaccine against one type or subtype of influenza virus confers limited or no protection against another type or subtype of influenza. Because the influenza virus undergoes continuous mutation, antigenic drift, antigenic shift, and reassortment with animal reservoir strains, creating new combinations of HA and/or NA proteins on the viral surface, yearly reformulation of the vaccine is required. There can be a mismatch between the viral strain present in the vaccine and that circulating, thereby reducing the effectiveness of flu vaccines. Furthermore, although current vaccines based on inactivated viruses are generally able to prevent illness in approximately 70-80% of healthy individuals under age 65, this percentage is far lower in the elderly or immuno-compromised subjects.
Currently, two classes of antiviral drugs are approved to treat influenza, neuraminidase inhibitors (e.g., Tamiflu® and Relenza®) and adamantane derivatives (e.g., amantadine and rimantadine). The neuraminidase inhibitors block the activity of neuraminidase (NA) surface protein and halt viral egress. The adamantane derivatives target the viral M2 protein and prevent the virus from uncoating and releasing its genetic material into the cell. However, there are increasing reports of emerging viral resistance to both classes of antivirals. Due to large scale resistance, the Centers for Disease Control and Prevention and others have recommended against using adamantane derivatives for the treatment or prophylaxis of Influenza virus. Many of the current anti-viral therapies are directed towards targeting viral components and are therefore prone to compensatory viral escape/mutation mechanisms.
Thus, there is an urgent need for alternative therapies to treat influenza and reduce the morbidity and morality associated with influenza infections.
PDE4 modulators potently inhibit TNF-α and IL-12 production, and exhibit modest inhibitory effects on LPS induced IL1β. See, e.g., L. G. Corral, et al., J. Immunol., 163: 380-386 (1999). PDE4 is one of the major phosphodiesterase isoenzymes found in human myeloid and lymphoid lineage cells. The enzyme plays a crucial part in regulating cellular activity by degrading the ubiquitous second messenger cAMP and maintaining it at low intracellular levels Inhibition of PDE4 activity results in increased cAMP levels leading to the modulation of LPS induced cytokines including inhibition of TNF-α production in monocytes as well as in lymphocytes.
A number of studies have been conducted with the aim of providing compounds that can safely and effectively be used to treat diseases associated with abnormal production of TNF-α. See, e.g., Marriott, J. B., et al., Expert Opin. Biol. Ther. 1(4):1-8 (2001); G. W. Muller, et al., Journal of Medicinal Chemistry, 39(17): 3238-3240 (1996); and G. W. Muller, et al., Bioorganic & Medicinal Chemistry Letters, 8: 2669-2674 (1998). Some studies have focused on a group of compounds selected for their capacity to potently inhibit TNF-α production by LPS stimulated PBMC. L. G. Corral, et al., Ann. Rheum. Dis., 58 (suppl I): 1107-1113 (1999). These compounds, often referred to as immunomodulatory compounds, show not only potent inhibition of TNF-α but also marked inhibition of LPS induced monocyte IL1β and IL12 production. LPS induced IL6 is also inhibited by immunomodulatory compounds, albeit partially. These compounds are potent stimulators of LPS induced IL10. Particular examples include, but are not limited to, the substituted 2-(2,6-dioxopiperidin-3-yl)phthalimides and substituted 2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindoles as described in U.S. Pat. Nos. 6,281,230 and 6,316,471. Monocyte/macrophage function is part of the Innate Immune System that serves as a first line of defense against an infection. By modulating the host's monocytes and macrophages, PDE4 modulators and immuno-modulatory compounds can change the dynamics of the response to a viral infection, such as influenza.
In addition, the innate immune system plays a critical role in the body's response to the influenza virus. The innate immune system relies on germline-encoded invariant pattern recognition receptors (PRR), such as Toll-Like Receptors (TLR), Nucleotide-binding domain and Leucine-rich-repeat Receptors (NLR), and Retinoic acid-inducible gene-1 Like Receptors (RLR). Activation of RLR, including the RIG-1-like receptors, by the signatures of influenza virus replication within the cytosol results in downstream activation of many nuclear transcription factors and the host's antiviral response. Dysregulation of the innate immune response by biological processes, or by a direct effect of the influenza virus, results in a poor immunological response by the host to the influenza virus. See, e.g., Samit R. Joshi, et al., Yale J. Biol. Med., 82(4): 143-51 (2009). The compounds provided herein may have an effect on the innate immune system in general, and on the RIG-1-like receptors specifically. See, e.g., M. R. Barber, et al., Proc. Natl. Acad. Sci. U.S.A., 107(13): 5913-18 (2010).